Chilling The Root Zone: Revolutionary Energy Cost-Saving Chilling System for Temperate Plant Production in Tropical ClimatesResearch studies at Temasek Polytechnic in Singapore show that controlled chilling of the root zone for temperate-climate vegetables can significantly increase vegetative growth and prevent them from wilting and bolting under tropical conditions. Temasek researchers have also developed a revolutionary cost-saving hydroponic chilling system that can be easily incorporated into existing commercial hydroponic systems with minimal capital inputs and disruption to existing infrastructures.

Agricultural crops are divided into cool-season and warm-season plants. Cool-season plants or temperate plants generally thrive poorly in the hot tropics. For example, butterhead lettuce, iceberg lettuce, cabbage and rhubarb are cool-season vegetables that tend to bolt in long day and high temperature environments.

Bolting is defined as the tendency of cool-season plants to grow rapidly and produce seed without first establishing strong vegetative growth when exposed to warm temperatures. Generally speaking, long days tend to favour bolting or seed stalk formation; while short days favour vegetative growth. Bolting results in numerous physiological and biochemical changes in plants. For example, the amount of bitter compounds, the sesquiterpene lactones, increase sharply as lettuce plants approach flowering. This causes off-flavour in lettuce, making the edible parts bitter and tough. Bolting also causes heat-sensitive head lettuce to form loose heads; this further renders the end products un-sellable.

In addition, high environmental temperatures can result in moisture stress, photosynthetic slum, and severe wilting in heat sensitive species, factors which can lead to a devastating loss in yield and productivity.

In temperate climate countries, growers adopt various planting strategies to combat or reduce the chance of bolting. One approach is to plant cool-season crops close together in wide rows to keep the soil and roots cool. Another approach is to avoid planting the crops too early in spring, as a sudden cold spell followed by warm weather can quickly set off bolting.

In the tropics, growers employ various evaporative cooling methods (e.g. the fan-and-pad method and fine mist fogging) to lower greenhouse temperatures, so as to create a cool greenhouse climate for cultivation of heat sensitive cool season plants.

In 1993, Dr Atomic Leow pioneered the use of thin film chilled nutrient solution or chilled nutrient mist to chill the root zone of cool season plants (e.g. zucchini, strawberry, tulips, carnations, cabbage and lettuce), and succeeded in raising these cool season plants in NFT (Nutrient Film Technique) channels and in the aeroponic system, respectively, in Singapore1. This pioneer work is fully detailed in a hydroponic textbook entitled A Guide to Hydroponics written by Dr Atomic Leow and published by the Singapore Science Centre in 1994. Subsequently, researchers and commercial growers in Singapore and in other parts of the tropics attempted to grow butterhead and iceberg lettuce using chilled nutrient solution technology with various degrees of success. For example, aeroponic farms in Singapore and Malaysia are using huge chillers to chill bulk quantities of nutrient solution, employing high-pressure pumps with timer control to spray either a continuous or intermittent chilled mist to the plant roots to grow a range of cool-season lettuce such as butterhead and romaine lettuce. However, this chilled solution technology is energy demanding and greatly increases the cost of production, which decreases the profit margin.

At Temasek Polytechnic in Singapore, Dr Atomic Leow invented a revolutionary energy cost-saving chilling system that can be easily incorporated into existing commercial hydroponic systems (NFT, aeroponics, and Deep Flow Technique), to deliver small quantities of chilled nutrient solution to the root zone of plants. This chilling technology enables many economically important temperate plants to be grown in the hot tropics at one seventh of the energy cost of the conventional chilled aeroponic system. This research paper provides convincing research evidence to show that using chilled solution technology, a range of difficult to grow cool season plants, such as purple basil, butthead lettuce, Shanghai Pak Choy and broccolini, can be successfully grown in tropical Singapore.

Effect of chilled solution temperature on root growth and plant development
Plant scientists have long observed that root zone temperatures exert a greater influence on the development of plants than the surrounding ambient air temperature 2,3. Crop species of tropical and sub-tropical origins, which are vulnerable to root zone chilling, are usually categorized as chilling-sensitive species. Some examples of chilling-sensitive species include cassava (Manihot esculenta), Ephedrine or Ma Huang (Ephedra vulgaris), cotton (Gossypium hirsutum), and mung bean (Vigna radiata). Species such as cucumber (Cucumis sativum), soybean (Glycine max), maize (Zea mays), common bean (Phaseolus vulgaris), and cereal grass (Sorghum spp.) are considered to be moderately sensitive species4, while the mustard family (Brassica spp.), barley (Hordeum vulgare), wheat (Triticum aestivum), and spinach (Spinacia oleracea) are classified as chilling resistant or tolerant species4.

Most published studies have investigated effects of root-zone chilling and plant responses using solution culture (e.g. 5, 6, 7, 8). Through these studies, it has been established that over-chilling of the root zone temperatures can have detrimental effects on the root and plant development, especially in chill-sensitive plants. For example, chilling had been shown to induce changes in the function and integrity of root cell plasma membranes; this may contribute to the associated decreases in water and nutrient (e.g. B, K+, Ca2+, NH4+, Cl-, NO3-) uptake by plant roots. Such changes include cell membrane lipid composition9, membrane fluidity10, and dysfunction of membrane-bound enzymes such as H+-ATPase11, 12.

In chilling sensitive species, cellular membrane alterations precede other cellular changes, and adverse effects on different cellular organelles are dependent on the duration of chilling and associated growth conditions, including photon flux density and relative humidity9. Root chilling can also cause a reduction in leaf stomatal conductance in a range of species, such as bean (Phaseolus vulgaris) 13, maize14 and tomato15. In chilling-sensitive species, chilling decreases root hydraulic conductance and slows down water absorption and impairs stomatal control that leads to excessive water loss and leaf wilting16,17. Chilling-tolerant maize lines, on the other hand, were found to have lower transpiration and higher water potentials in recently matured leaves than those in chilling-sensitive lines18.

In short, over-chilling of the root zone can result in tissue damage, interference in ion transport, and impair leaf stomatal control. Thus, growers who intend to exploit chilled nutrient solution technology to grow cool-season crops in the hot tropics, must first take into consideration the nature of the cool-season crops under study, and to experimentally determine the upper and lower range of chilled solution temperatures that can be applied to the root zone of plants under study for prolong periods without incurring tissue damage and physiological dysfunction to the plants.

In this research report, the authors conducted mass screening of some selected cool season plants for heat tolerance. The upper and lower range of chilled solution temperatures that can be fed to the root zone of these selected heat-tolerant varieties were experimentally determined, and then tested in the energy cost-saving chilling system developed at Temasek Polytechnic for small-scale commercial trials in the hot tropics.

Comparative growth studies of sweet and purple basils raised by five different types of hydroponic treatmentsMaterial and Methods
Seedlings of sweet and purple basil (Red Rubin) were transplanted into either a Deep Flow Water Culture (DFWC) or Nutrient Film Technique (NFT) growth system when they attained a height of about 2cm.

In DFWC, the basil roots were immersed in a large quantity of either chilled re-circulating nutrient solution (Treatment 5) or non-chilled re-circulating nutrient solution (Treatments 1 & 2). For NFT, the basil roots were continuously fed with a thin film of either chilled re-circulating nutrient solution (Treatment 4), or non-chilled re-circulating nutrient solution (Treatment 3). For treatments involving chilled solution, chilling was carried out entirely using the energy cost-saving chilling system developed at Temasek Polytechnic. The five experimental treatments were summarized as follows:

The leaf areas and height of the basil plants grown in the five different hydroponic treatments were measured weekly and the data collected were subjected to the Duncan Multiple Range Test (DMRT) to establish the degree of statistical significance.

Results and Discussions
The results of Table 1 and Table 2 show that sweet basil plants could be grown relatively easily in Singapore using NC-DFWC. However, massive root rots were observed in the sweet basil plants on day 40 of cultivation. The heat-sensitive purple basil plants also displayed a similar growth response when raised in NC-DFWC, with massive root rot set in by day 40 of cultivation (see Fig. 1). As root rot was not observed in the purple basil plants raised by NC-NFT, it was concluded that prolonged exposure of the root zones to large amounts of warm (30°C), non-chilled nutrient solution, was detrimental to the basil root development and was directly responsible for the observed root rots seen in NC-DFWC. Generally, purple basil plants raised in NC-NFT performed significantly poorer than those raised in NC-DFWC, albeit the advantage of root rot absence.

Figure 1. Massive root rots of un-chilled purple basil plants.

Chilled nutrient solution (26°C) confers a definite protective role to the basil roots. By growing the purple basil in C-NFT and C-DFWC, the root rot problem could be completely circumvented (see Fig. 2). Chilling also greatly enhanced the vegetative growth of the purple basil plants (see Fig. 3 and Fig. 4), and intensified the purple anthocyanin of the purple basil plants (see Fig. 4). This enhancement in vegetative growth and intensification of the purple colouration were related to the amount of chilled nutrient solution being fed to the plant roots.

A large amount of chilled nutrient solution, as in C-DFWC, was shown to be more effective than a shallow amount of chilled nutrient solution, as in C-NFT. In addition, chilling was also effective in removing the bitterness experienced in non-chilled purple basil plants.

Unpublished data of Dr Atomic Leow and Ms Ruth Wong had shown that chilling of basil roots significantly increases the methyl chavicol content of the purple basil plants. Perhaps, this may explain why chilling was effective in removing the bitterness experienced in non-chilled purple basil plants.

These comparative growth studies categorically establish that the temperate purple basil plants could be grown commercially in the hot tropics using C-NFT or C-DFWC. The latter was the preferred method as it results in a marked increase in vegetative yield, intensification of the purple colouration, and complete removal of the bitterness experienced in the untreated purple basil plants. The optimal solution temperature for growing purple basil in the hot tropics was found to be in the region of 25°C – 26°C. Solution temperatures lower than 20°C could result in irreversible damage to the basil roots.

Comparative growth studies of butterhead lettuce raised by six different types of hydroponic treatments

Materials and Methods
Seedlings of butterhead lettuce (Rex) were transplanted into either a Deep Flow Water Culture (DFWC), Nutrient Film Technique (NFT) channels, or the aeroponic system when they attained a height of about 2cm. The hydroponic systems were supplied with chilled or non-chilled nutrient solution. For treatments involving chilled solution, chilling was carried out entirely using the energy cost-saving chilling system developed at Temasek Polytechnic. The six experimental treatments are summarized as:

The leaf area of butterhead lettuce was measured weekly and the data collected were subjected to the Duncan Multiple Range Test (DMRT) to establish the degrees of statistical significance.

Results and Discussion
The results of Table 3 show that C-DFWC and C-Aeroponics were far more superior than C-NFT in promoting leafy growth of butterhead lettuce (Rex). In addition, butterhead lettuce raised in C-DFWC and C-Aeroponics also formed better semi-loose heads than those raised by C-NFT.

Of the three types of chilling systems investigated, C-DFWC was by far the best system for growing heat-sensitive butterhead lettuce in the tropics. Butterheads raised in C-DFWC (Fig. 6) was also found to weigh 25% more than those raised in C-Aeroponics.

Dr Atomic Leow
Without chilling the root zones, butterhead lettuce performed poorly in the hot tropics (see Fig.6). Figure 7 and Figure 8 show the close views of two varieties of butterhead lettuce (Panama, Rex) successfully raised in Singapore by C-DFWC.

Figure 7. Close view of butterhead lettuce (Panama) raised in C-DFWC.

Figure 8. Close view of butterhead lettuce (Rex) raised in C-DFWC.

Extra chilling of the root-zone temperature to 20°C, as in Treatment 6 using the EC-DFWC, conferred no added advantage, but was actually detrimental to the growth of the butterhead lettuce (Table 3, Fig. 6).

In conclusion, production of high quality temperate butterhead lettuce in tropical Singapore is no longer a myth, and can be achieved by growing a heat-tolerant variety butterhead in combination with an energy cost-saving C-DFWC, like that devised at Temasek Polytechnic.

Materials and Methods
Seedlings of Shanghai Pak Coy (Brassica campestris Chinensis) were transplanted into either a NC-DFWC or C-DFWC when they attained a height of about 2cm. For the latter treatment, the nutrient solution was chilled using the energy cost-saving chilling system developed at Temasek Polytechnic.

The leaf areas of the Shanghai Pak Coy were measured weekly and the data collected were subjected to Duncan Multiple Range Test (DMRT) to establish the degrees of statistical significance.

Results and Discussions
The results of Table 4 show that heat sensitive Shanghai Pak Choy (Brassica campestris Chinensis) performed poorly in NC-DFWC in Singapore. The plants were grossly under-sized and displayed severe heat distress responses such as wilting and folding-up of the leaves on hot afternoons (see Fig. 7).

In contrast, Shanghai Pak Choy grew robustly in C-DFWC and was darker green in colour and showed no signs of any wilting or bolting under the scorching tropical sun (Fig. 8 and Fig 9). Thus, this study shows that heat sensitive Brassica species can be grown successfully in hot tropics by mere manipulation of the nutrient solution temperature bathing the root zone by a few degrees downward.

Figure 9. Shanghai Pak Choy performed poorly in NC-DFWC.

Figure 10 . Pak Choy displaying vigorous
vegetative growth in C-DFWC.

Figure 11. Close view of the Shanghai Pak Choy raised in C-DFWC.

The amazing effect of root-zone chilling to promote vegetative growth of heat-sensitive vegetables raised in the hot tropics was elegantly demonstrated in 1993 by Dr Atomic Leow when he transformed a dwarf variety of Pak Choy into a gigantic size merely by supplying a shallow stream of appropriately chilled nutrient solution to the root zone of plants (see Fig. 12). Unpublished data by Dr Atomic Leow while working with overseas researchers in 1996 indicates that chilling of the plant root zone increases the gibberellin-like substances in plants by several folds.

Figure 12. Comparison of the size of dwarf Pak Choy raised in NC-NFT (held up by Dr. Leow) and in C-NFT.

Conclusion
In summary, Temasek Polytechnic of Singapore have invented a revolutionary energy cost-saving hydroponic chilling system that can be easily incorporated into existing commercial hydroponic systems with minimal capital inputs and disruption to existing infrastructures. The hydroponic chilling system is unique in that it delivers only a small quantity of pre-determined chilled nutrient solution to the root zone to enable a range of heat-sensitive, cool-season plants to be cultivated in Singapore. Being an energy cost-saving system, the chilling system consumes only one-seventh of the energy cost of conventional chilled aeroponics.

Research studies conducted at Temasek Polytechnic also show that controlled chilling of the root zone of heat-sensitive vegetables in the tropics can significantly increase the vegetative growth of vegetables and prevent plants from wilting and bolting under the high tropical heat.

Unpublished data by Dr Atomic Leow while working with overseas researchers in 1996 indicated that chilling the root zone causes the gibberellin-like substances of plants to rise by several fold. Chilling also intensifies the anthocyanin pigmentation of purple basil plants, the chlorophyll pigmentation of heat-sensitive Brassica species, and prevents the photo catalytic breakdown of these pigments under extreme tropical heat.

Of the various hydroponic systems tested at Temasek Polytechnic, C-DFWC was found to be superior for the cultivation of heat-sensitive, cool-season plants in the tropics, as shown by the robust growth of broccolini plants (Fig. 13 and Fig. 14). It is anticipated that this ground-breaking energy-saving technology developed at Temasek Polytechnic will generate tremendous interest for both local and foreign investors for the cultivation of high-value temperate crops in the hot tropics.

Figure 13. Robust broccolini plants growing in C-DFWC.

Figure 14. A close view of broccolini plant growing in C-DFWC.

Acknowledgements
The authors would like to thank Temasek Polytechnic and the Singapore Totalisator Board for making available the funds and manpower needed for this research study.

By Steven Carruthers
Following a stellar career as one of Singapore’s foremost hydroponic research scientists and tertiary educators, Dr Atomic Leow Chuan Tse has resigned as Head of the Biotechnology Specialist Unit, Temasek Applied Science School, at Temasek Polytechnic in Singapore, to pursue his life-long dream to study medicine at the University of East Anglia in the United Kingdom.

Among Dr Leow’s many industry achievements has been the development of a revolutionary hydroponic system to produce spectacular commercial cut orchids. The ‘Precise Influx Hydroponic Growth System’ (PIHGS) utilizes an automated mechanism to deliver precise, pre-determined amounts of nutrient solution that effectively circumvents the root-rot problem faced by many hydroponic orchid growers. (Many of our readers will recall our feature story on this revolutionary system in our January/February 2006 issue.) The technology is now licensed to a biotechnology company and a major orchid grower in Singapore. Dr Leow is also the inventor of a revolutionary, cost-saving chilling system for temperate plant production in the tropics, featured in this issue.

Formerly educated in Australia, Dr Leow holds a Bachelor of Agricultural Science (Honours) and Diploma in Education from La Trobe University. He is also a Doctor of Philosophy (Toxicology), the result of a La Trobe University Research Scholarship for PhD research in toxicology of drugs.

In 2006, he was awarded a Graduate Diploma in Christian Studies (summa cum laude and Dean’s Award).

Between 1982 and 1990, while a biology lecturer at St Andrew’s Junior College in Singapore, Dr Leow pioneered the first Bodybuilding Club in Singapore, an interest that stems from his high school years when he was placed second in the Mr Chung Ling Muscle Improvement Competition in Penang, Malaysia. In 1985, he was awarded the Certificate of Merit by the Asia Bodybuilding Federation, and the Certificate of Merit by the International Federation of Body-Builders (IFBB), Montreal, Canada (1986).

Returning to his academic career, in 1984 and again in 1987, Dr Leow supervised the science students to capture first prize in the Singapore National Science Fair. In 1990, he became one of the pioneer biotech lecturers that helped to start up the Biotechnology course at Ngee Ann Polytechnic in Singapore, where he lectured in plant tissue culture, animal and plant cell biology, hydroponics and biochemical laboratory techniques. Between 1992 and 1997, Dr Leow was a senior lecturer and Agrotechnology Coordinator at Singapore Polytechnic, where he lectured in Plant Cell Biology, Human Anatomy and Physiology and Agrotechnology, as well as teaching inorganic chemistry to biotechnology students.

In 1995, Dr Leow authored the world’s first CD-Rom on hydroponics, which won the ‘Creative Ngee Ann Multimedia Award’. During this period he also mentored the gifted students in the top Secondary Schools in Singapore on ‘Life Sciences’ research projects. In 1996 he was invited as a Visiting Fellow to the prestigious Institute of Agribiology & Soil Fertility at the University of Wageningen in the Netherlands, where he presented a seminar paper on the ‘Impact of hydroponic technology on the agrotechnology industries in Singapore’ and the ‘Effect of nutrient cooling on the gibberellins metabolism in Simposai’. During the 1990’s, Dr Leow also served as a volunteer counselor for Singapore Prisons and Drug Rehabilitation Centres.

Between 1997 and 2006, Dr Leow has been the principal lecturer at Temasek Polytechnic and Head of the Biotechnology Specialist Unit where he secured international accreditations from numerous international universities for the ‘Diploma in Biotechnology’ programme at Temasek Polytechnic.

Although Dr Leow had fulfilled all the requirements to study medicine as a younger man, he was unable to do so owing to his deep commitment to raise his two daughters with care and love. His youngest daughter is now a fifth year medical student at the School of Medicine at Melbourne University. In fact, medicine seems to run in the family with a brother-in-law the Clinical Director at Frankston Hospital, Victoria, and a cousin and nephew who also have careers in medicine. Now the time has come for Leow to follow his life-long passion to study medicine.

As a strong advocate of hydroponic technologies, and a role model for his many students and colleagues, Leow’s contribution to the industry has been enormous and our best wishes go with him in his new career where I am certain he will make an indelible mark. Farewell old friend. The hydroponics industry is the poorer for your departure. Ω